1. Field of the Invention
The present invention relates to a monochromator capable of picking up light of a desired wavelength in high precision without being influenced by changes in ambient temperature. The invention also relates to an optical spectrum analyzer equipped with the monochromator.
2. Description of the Related Art
FIG. 3 shows an example of the conventional optical spectrum analyzer. The optical spectrum analyzer generally indicated at 100 comprises a monochromator 110 of the Czerny-Turner dispersion type, an analysis and control section 120 which not only controls the monochromator 110 but also analyzes the result of spectroscopy with the monochromator 110, and a display section 130 for displaying the result of measurement.
The monochromator 110 basically comprises an entrance slit 111 through which the incident light is passed from a light source 200, a concave mirror 112 by which the light passing through the entrance slit 111 is converted to parallel light and reflected, a plane diffraction grating 113 for diffracting the parallel light from the concave mirror 112, a concave mirror 114 for reflecting and condensing part of the diffracted light from the plane diffraction grating 113, an exit slit 115 through which the reflected light from the concave mirror 114 passes to emerge, and a photo detector 116 which measures the intensity of the emerging light from the exit slit 115 and which outputs it to the analysis and control section 120.
Thus, the monochromator 110 condenses that part of the diffracted light from the plane diffraction grating 113 which is directed toward the concave mirror 114 and then measures the intensity of that light.
The plane diffraction grating 113 is driven to rotate by a motor 113a so that it alters the wavelength of the light being diffracted toward the concave mirror 114. The motor 113a is controlled by the analysis and control section 120. Thus, the analysis and control section 120 controls the angle of the plane diffraction grating 113 via the motor 113a, thereby controlling the wavelength of the light being received by the photo detector 116. In other words, the analysis and control section 120 measures the intensity of light of a specified wavelength by setting the angle of the plane diffraction grating 113 to a specified value.
All members of the monochromator 110 including the substrate will expand and shrink thermally. Hence, the position of the concave mirror 114 relative to the plane diffraction grating 113 changes subtly with the ambient temperature and there has been the potential for the failure to separate light of a specified wavelength even if the angle of the plane diffraction grating 113 is set to a specified value.
An object, therefore, of the present invention is to ensure that light of a specified wavelength is separated in high precision despite changes in the ambient temperature.
To achieve the above object of the invention, there is provided a monochromator (10) comprising a diffraction grating (e.g. plane diffraction grating 13) for diffracting incident light and a selection means (e.g. concave mirror 14) for selecting light of a desired wavelength from the light diffracted by said diffraction grating, characterized in that said diffraction grating is formed of a material selected such that a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of members other than said diffraction grating is cancelled out or reduced by a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of said diffraction grating.
The diffraction grating also expands and shrinks thermally as temperature changes. Upon expansion or shrinking, the grating constant of the diffraction grating changes and so does the diffraction angle of light of the same wavelength. Hence, thermal expansion or shrinkage of the diffraction grating causes variations in the light selected by the selection means. The amount or direction of such variations can be adjusted by altering the constituent material of the diffraction grating.
Hence, by choosing an appropriate constituent material for the diffraction grating a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of members other than the diffraction grating can be cancelled out or reduced by a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of the diffraction grating. As a result, there is provided a monochromator that can separate light of a specified wavelength in high precision despite changes in ambient temperature.
According to the present invention, there is provided a monochromator (10) comprising a diffraction grating (e.g. plane diffraction grating 13) for diffracting incident light and a selection means (e.g. concave mirror 14) for selecting light of a desired wavelength from the light diffracted by said diffraction grating, characterized in that said diffraction grating is formed of a material selected such that a variation in the wavelength of the selected light that occurs under the same selection conditions on account of a change in the layout of individual members due to thermal expansion or shrinkage of the substrate for fixing the individual members is cancelled out or reduced by a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of said diffraction grating.
According to invention, there is provided a monochromator that relies upon the same principle to separate light of a specified wavelength in high precision despite changes in ambient temperature.
The monochromator may be of the Czerny-Turner dispersion type. The constituent material of the diffraction grating may be PYREX(trademark)-brand borosilicate glass.
According to the present invention, there is provided an optical spectrum analyzer (1) equipped with the monochromator above mentioned.
According to the invention, light of a specified wavelength is separated and its intensity measured in high precision irrespective of changes in ambient temperature.